Handover (HO) means that a mobile station moves from air interface of a base station to air interface of another base station. A handover process in the conventional IEEE 802.16e system is described below.
In an IEEE 802.16e network, a serving base station (SBS) can broadcast neighbor base station information through a neighbor advertisement (MOB_NBR-ADV) message to inform a mobile station (MS) of information (topology) about fundamental network configuration.
The MOB_NBR-ADV message contains system information on the serving base station and neighbor base stations, for example, a preamble index, frequency, a degree of hand-over optimization, downlink channel descriptor (DCD)/uplink channel descriptor (UCD) information, etc.
The DCD/UCD information includes information that the mobile station should know in order to transmit/receive information through a downlink and an uplink. For example, the DCD/UCD information includes hand-over trigger information, MAC (Medium Access Control) version of a base station, MIH (Media Independent Hand-over) capability, etc.
More specifically, the DCD includes downlink burst profiles (DL_Burst_Profile) that is information used for the MS to decode a message transmitted from the BS. Since it is desirable that all MSs in the coverage of the corresponding BS can receive messages broadcasted by the BS, DL_Burst_Profile set value that is most robust to error is applied. Thus, the set value is barely changed. However, a message unicast-transmitted to each MS is sent as a set value suited to the channel state of each MS, and thus different DL_Burst_Profile set values may be sent to different MSs in case of unicast transmission. The UCD includes UL_Burst_Profiles used for the MS to transmit messages to the BS and may be set depending on a channel environment state.
As described above, in the conventional IEEE 802.16e system, system information is transmitted through the UCD of uplink and the DCD of downlink and may have a transmission period of a maximum of 10 seconds. At this time, when the system information is updated, the BS schedules such that all MSs recognize the updated system information at least once so as to prevent mismatch between system information possessed by the MSs and the system information of the base station. To achieve this, for the UCD, a period of UCD transition interval start and transition interval expired is defined and the BS transmits a new UCD in the period.
A process of enabling an MS which acquires system information of neighbor BSs through the aforementioned method to perform a hand-over in an IEEE 802.16e network is described in more detail.
A hand-over procedure in the conventional IEEE 802.16e may consist of three processes of hand-over initiation and preparation, hand-over execution, and hand-over completion.
An exemplary fundamental hand-over procedure which can be configured as above is explained with reference to FIG. 1.
FIG. 1 illustrates an exemplary hand-over procedure which can be performed in an IEEE 802.16e system.
Referring to FIG. 1, an MS may be linked to a serving base station (SBS) and exchange data with the SBS (S101).
The SBS may broadcast information on neighbor BSs to the MS through a MOB_NBR-ADV message (S102).
The MS may start to scan candidate hand-over base stations (HO BSs) using a hand-over trigger condition while communicating with the SBS. The MS may transmit a hand-over request (MOB_MSHO-REQ) message to request the SBS to perform a hand-over when a hand-over condition is satisfied, for example, when a predetermined hysteresis margin value is exceeded (S103).
The SBS may inform the candidate HO BSs corresponding to information included the MOB_MSHO-REQ message of hand-over request from the MS through an HO-REQ message (S104).
The candidate HO BSs may perform pre-processing for the MS that requests the hand-over and transmit information about the hand-over to the SBS through an HO-RSP message (S105).
The SBS may transmit the information about the hand-over, acquired through the HO-RSP message, from the candidate HO BSs to the MS through a MOB_BSHO-RSP message. Here, the MOB_BSHO-RSP message may contain information required to perform the hand-over, such as action time, hand-over identifier (HO-ID), a dedicated HO CDMA ranging code, etc.
The MS may determine one target BS from among the candidate HO BSs on the basis of the information included in the MOB_BSHO-RSP message received from the SBS. Accordingly, the MS can attempt to perform ranging by transmitting a CDMA code to the determined target BS (S107).
Upon reception of the CDMA code, the target BS may transmit information representing whether or not ranging is successfully performed and physical correction values through a RNG-RSP message (S108).
Subsequently, the MS may transmit a ranging request (RNG-RSP) message for authentication to the target BS (S109).
Upon reception of the ranging request message from the MS, the target BS may provide system information that can be used in the target BS, such as CID (Connection Identifier), to the MS through a ranging response message (S110).
When the target BS has successfully performed authentication of the MS and transmitted all update information, the target BS may inform the SBS of the MS whether the hand-over is successfully performed through a hand-over completion (HO-CMPT) message (S111).
The MS can exchange information with the target BS to which the hand-over has been performed (S112).
For an IEEE 802.16m system, names and/or functions of medium access control (MAC) management messages used in the above-described hand-over procedure are changed.
A hand-over procedure which can be performed in the IEEE 802.16m system is similar to the above-described hand-over procedure of the IEEE 802.16e system. However, names and/or functions of MAC management messages are changed as follows.
MOB_NBR-ADV→AAI_NBR-ADV: this message includes system information transmitted in the form of an S-SFH not DCD/UCD.
MSHO-REQ→AAI_HO-REQ
BSHO→AAI_HO-CMD
RNG-REQ (CDMA code)→Ranging preamble code
RNG-RSP (ranging status)→AAI_RNG-ACK (ranging status)
RNG-REQ (MAC message)→AAI_RNG-REQ
RNG-RSP→AAI_RNG-RSP: this message includes a station identifier TSTID or STID instead of CID
Furthermore, in the IEEE 802.16m system, system information of a BS is transmitted through a superframe header.
A frame structure and a superframe header of the IEEE 802.16m system will now be described.
FIG. 2 illustrates an example of a physical frame structure used in a wireless wide area network (WAN) mobile communication system based on an IEEE 802.16 system.
Referring to FIG. 2, a superframe has a length of 20 ms and is composed of four frames.
One frame includes eight subframes. The eight subframes may be divided into a downlink subframe region and an uplink subframe region including a predetermined number of subframes according to a downlink-to-uplink (DL/UL) ratio. When the UL/DL ratio is 5:3, as shown in FIG. 2, five subframes from among the eight subframes are allocated to downlink subframes SF0 to SF4 and the remaining three subframes are allocated to uplink subframes SF5 to SF7.
An idle time in which data symbols including data are not allocated, that is, a TTG (Transmit/receive Transition Gap) is present between the downlink subframe region and the uplink subframe region. Furthermore, an idle time, that is, an RTG (Receive/transmit Transition Gap) may follow the downlink subframe region. One subframe is composed of six OFDM symbols.
A BS and an MS may exchange data using the above frame structure. For example, the MS can receive data from the BS using downlink subframes and transmit data to the BS using uplink subframes. On the other hand, the BS can transmit data to the MS using downlink subframes and receive data from the MS using uplink subframes.
A superframe header can be transmitted to the MS through a first subframe of the superframe in the above frame structure. The superframe header may include resource allocation information composed of frames or subframes included in the superframe header or system information.
More specifically, in the IEEE 802.16m system, the superframe header (referred to as “SFH” hereinafter) may include essential system parameters, system configuration information, etc.
The SFH can be divided into a primary superframe header (P-SFH) and a secondary superframe header (S-SFH). The P-SFH transmitted for each superframe includes four least significant bits of a superframe number and information about an S-SFH transmitted in the corresponding superframe. The S-SFH transmits actual system information. The system information is divided into subpackets depending on the property thereof, and the subpackets are referred to as S-SFH SPn (n=1, 2, 3). S-SFH SP IEs are transmitted in different transmission periods TSP1<TSP2<TSP3. Here, the system information is set values depending on communication environment such as ranging, power control, etc., which are required for the MS to perform downlink/uplink (DL/UL) transmission.
The information about the S-SFH, included in the P-SFH, may contain an S-SFH change count which represents the version of a currently transmitted S-SFH, an S-SFH scheduling information bitmap which represents whether or not the S-SFH is transmitted in the corresponding superframe, an S-SFH size which represents the number of logical resource units allocated for S-SFH transmission, an S-SFH number of repetitions, which represents the transmission format of the S-SFH, an S-SFH SP change bitmap which represents a changed S-SFH, etc. Here, field sizes of the S-SFH scheduling information bitmap and S-SFH SP change bitmap correspond to a total number of S-SFH SPs included in the corresponding superframe.
When the MS performs a hand-over to a specific target BS, a hand-over delay time may vary according to whether SFH information of the target BS, which is possessed by the MS, is updated or not. If the hand-over is performed in a state in which the MS has received an SFH of the target BS through an AAI_NBR-ADV message, the MS can directly perform network re-entry in the target BS to complete the hand-over.
However, the MS may not receive the AAI_NBR-ADV message including the SFH of the target BS or may receive an AAI_NBR-ADV message in which a change in the SFH of the target BS is not reflected yet. In this state, the MS which performs the hand-over receives all SFHs from the target BS before performing network re-entry. That is, the hand-over delay time increases by a time required for the MS to receive all S-SFH SP1, SP2 and SP3. In addition, hand-over optimization such as dedicate ranging using a dedicated ranging code or seamless hand-over cannot be performed.
In other words, if the MS cannot all the SFHs of the target base station or directly receives SFHs which are not updated from the target BS during the hand-over, the hand-over delay time may remarkably increase, and thus a minimum hand-over delay time specified by standardization standards may not be satisfied.